Run process(es) under supervision

This runs the processes defined in a Procfile using the daemontools supervisor. This has some advantages:

  * Multiple processes can be started
  * Restarting failed processes
  * Logfile rotation (defaults to 10 files of 1MB each)

Defining which processes get started and how many of each is determined by the parameter /start is being called with.

Step one is extracting a `Procfile` from `.release` if none exists. In this case `/start` defaults to running the `web` process. Otherwise it defaults to starting one of each process defined in the `Procfile`.

If `/start` is being called with a process name like `web` as parameter only that process will be started.

If `/start` is being called with a `formation` like `web=1,worker=2,clock=1` it will start 1 web, 2 worker and 1 clock process. Processes not named in the formation wont get started.

Currently starting more than 1 web process will almost certainly will wreak havoc on your container since you only have a single port to listen on.

The exporter is based in large part on [Foreman](https://github.com/ddollar/foreman) and modified to fit the needs of this special application.